The present invention relates to a process for preparing a planographic printing plate by electrophotographic or electrographic means. Specifically, the process comprises producing a charge image on a photoconductive or highly insulating layer, developing this image on the free surface of a dielectric intermediate support in contact with the photoconductive or highly insulating layer through use of a developer, transferring the toner image to a planographic printing plate, and fixing the toner image.
There has been described in U.S. Pat. No. 2,990,278 a process wherein an electrophotographically produced toner image is transferred by means of a corona from a photoconductor cylinder to a continuous intermediate support web. The toner image is heated on the web until it becomes tacky, whereupon it is transferred to the final image support. The intermediate support web may consist of polytetrafluoroethylene, an abhesive plastic sheet or a glassy material. The final image support is a roughened aluminum or zinc plate of the type preferably used in printing.
U.S. Pat. No. 3,554,836 discloses a device wherein at least one monolayer of a developer powder is transferred imagewise as a toner image to a continuous tape which is rolled around several rolls to form an intermediate support having a silicone elastomer surface. An infrared lamp is situated at the underside of the web which is transparent to radiation. The developing powder is directly heated by the radiation absorbed by the web. It is also possible to heat the intermediate support web in its entirety, so that the powder is softened by contact heat. By means of another roll, the sheet-shaped image support is brought into contact with the developer powder on the web so that the powder is transferred to the image support. The image support used is, for example, aluminum.
In both processes the toner image is tranferred twice, namely, once from the photoconductor surface to the intermediate support, and then from the intermediate support to the final image support. In the course of these transfers, image resolution suffers.
There is also known an electrostatographic imaging process (German Patent No. 2,200,084, equivalent to U.S. Pat. No. 4,027,964) wherein a photoconductive recording material is covered with a thin dielectric web prior to the development of the charge image. The developer liquid is then applied to the web, with the toner image obtained being transferred to the final image support. The latter step is performed before separating the dielectric tape from the recording material and is necessary to prevent physical or electrostatic distortion of the toner image. The process can only employ a polar developer liquid which is applied by means of an applicator element, such as a roll, whose surface is evenly patterned with elevations and recesses. The dielectric intermediate support web is 3-75 .mu.m thick, and preferably consists of a polypropylene or polyvinyl fluoride film. The toner image is transferred to the final image support by means of pressure and/or with the aid of an electric voltage bias. The image support mentioned is primarily ordinary paper. Of special interest is electrophoretic developing with negatively charged toner particles, instead of developing with a polar developer liquid. An applicator roll is used which has a smooth surface and which is in even contact with the film of liquid. However, the very low density of the toner images thus prepared renders the process unsuitable for practical use.
German Offenlegungsschrift No. 2,418,240 describes an electrophotographic copying process wherein an electrostatic image is developed with a liquid developer on a final web-shaped image support, in the form of a thin film, by bringing the reverse face of the image support, the front of which is lying flat against the photoconductor cylinder carrying the electrostatic image, into contact with the liquid developer. In this process, the electrostatic field of the charge image penetrates through the thin film, as a consequence of which the toner particles dispersed in the liquid developer can migrate in the direction of the photoconductor cylinder. The particles adhere to the reverse face of the film, and form the fixable toner image. Although this method advantageously results in the use of an uncoated thin image support, the elimination of the cleaning of the residual developer from the photoconductor surface and possibly even the preparation of several copies from one charge image, the handling of the thin film as an image support proves to be highly problematic and leads to considerable technical difficulties.
German Offenlegungsschrift No. 2,125,013 discloses a process for the production of copies, in which an electrostatic charge image is developed by a toner with the toner image being transferred to a final support. The process involves the steps of covering the induced charge image with an insulating film and developing the induced charge image with a toner, pressing the final support against the toner image-carrying film, removing the film and final support from the toner image, and finally separating the film and the final support. This process has the disadvantage that, before the film is removed from the charge image, the final support must be applied onto the toner image-carrying film, otherwise, the toner image will be destroyed before the final support will have been applied to the film.
Further, a process for producing copies by applying an electrostatic charge is described in German Offenlegungsschrift No. 2,125,050. In this process, development is performed by means of a liquid, conductive toner. The electrostatic charge image is covered with a thin film, and the charge image is transferred to the film by induction and developed with the toner. Thereafter, the film is dyed with a toner-repellant dyed substance, while the toner image is still wet, and the liquid is transferred to a suitable support, using a method known in offset printing. Processes of this kind have been found to be rather complicated, however, and therefore, have not gained general acceptance.